Microwave oscillator



1956 H. v. NEHER MICROWAVE OSCILLATOR Filed Nov. 26, 1945 FIG. 2

INVENTOR HENRY V. NEHER ATTORNEY nited States Patent MICROWAVEOSCILLATOR Henry V. Neher, Pasadena, Calif., assignor, by mesneassignments, to the United States of America as represented by theSecretary of the Navy Application November 26, 1945, Serial No. 630,947

Claims. (Cl. 315-42) This invention relates to transit time microwaveoscillators.

Desirable characteristics for such oscillators may include thefollowing: (1) low plate voltage, (2) low power for heating cathode, (3)simplicity of construction, (4) stability of operation and insensitivityto temperature changes, (5) wide range tunability, (6) constant poweroutput over wide band of frequencies, and (7) good efiiciency. Widerange and constant output appear to be incompatible with high efficiencyfor low power tubes. However, the present invention'provides a tube inwhich the characteristics of low plate voltage, low cathode heaterpower, simplicity of construction and high eificiency can be obtained toa degree substantially equivalent to that achieved with conventionaltubes operating in the long Wave length region. in particular, forexample, a tube in accordance with this invention may operate in the 10cm. region with a cathode supply of 1.5 volts at 0.25 amp.; a platesupply of 100 volts, plate current of 10 ma., and deliver 0.1 to 0.2watt of power. Such a tube would have many applications.

It is therefore the object of this invention to provide a transit timemicrowave oscillator of simple, sturdy construction which will operatewith low plate voltage, small cathode supply voltage and with highefficiency.

I Another object is to provide a transit time oscillator tube which isfrequency tunable by either temporary or permanent deformation of theouter envelope.

Other features, characteristics and advantages of the invention willappear from the following detailed description when taken together withthe drawings the figures of which are illustrative of the principles ofthis invention which should not be deemed to be limited except by thespecification and the spirit of the appended claims.

Fig. l is a middle cross section showing the anode positions, Fig. 2 isan internal view also showing the cavity and anode positions therein,Fig. 3 is a diagram showing the relative positions of the variouselectrodes and Fig. 4 is a typical embodiment showing the location andrelation of the essential elements of the tube.

Referring now to Fig. 1 there is shown a metal cylinder 1 having twoanode vanes 3 and 5 mounted on the inner circumference at opposite endsof a diameter. If the spacing between the adjacent ends of the vanes 3and 5 is, say 0.1 of the diameter of the cylinder 1 and if theirthickness is equal to their separation, then this section of thecylinder 1 can be made to resonate in the fundamental mode, as shown, ata wave length about three times the diameter of the cylinder 1.

Fig. 2 also shows a cylinder 7 with the anode vanes 9 and 11 and thefundamental modes H and E lines.

Radiation is prevented from leaking out the ends of the cylinder 7because at. this wave length the circular guide is beyond cutoff and hasan attenuation of about 30 db for each diameter distance along thecylinder toward an end. V

Fig. 3 shows a cathode 13 in the form of a hairpin mounted midwaybetween two anode vanes 15 and 17,

2,736,839 Patented Feb. 28, 1956 ice and between two pairs of grids, 19and 21, and 23 and 25, mounted respectively between cathode 13 and anodevane 15 and between cathode 13 and anode vane 17. Let the grids 21 and23 nearest the cathode 13 be kept at a small direct current potentialwith respect to the cathode. At one phase of the radio frequency cyclethe electric field might be as shown in Fig. 1. It will be seen that aradio frequency voltage will be built up across the cathode-grid orinput circuits which will be in phase with the voltage in the main anodecircuits. The voltage gradient will be divided among the various gapsaccording to the relative reactances of the gaps, hence most of thevoltage will be across the gap between anode and outer grid since thecapacity across this gap will be normally only about one-fifth of thecapacity across cathodegrids. Thus radio frequency voltage built upbetween the cathode and the first two grids supplies the power fordriving the input circuit. In order to achieve a uniform field betweencathode and grid 21 filamentary type may be used or a fiat solid cathodemay be used though the latter is not suitable for battery operation.This will keep the phases of electrons passing through the grids thesame.

The phase of the radio frequency voltage is such as first to ejectcurrent through grid 21, and then on the other half cycle through grid23. These bunches of current must arrive in the regions between thesecond grids 19 and 25.respectively and their respective adjacent anodes15 and 17 when the anodes are farthest negative with respect to theiraverage potential, in order to deliver the most power into the anodecircuit. It is evident then that there must be a phase difference of 11'radians (or 21r+1r or 4rr-I-1r etc.) between the radio frequency voltageacross the input circuit and the arrival of the current in the outputcircuit. This can be obtained by adjusting the spacing between therespective pairs of first and second grids, that is, between 21 and 19and between 23 and 25, and by properly choosing the potential of theouter grids 19 and 25.

For such a tube operating on 100 volt maximum, in the 10 cm. region, itis not easy to keep the total transit angle down to 1r radians. In factthere will be a phase shift of about 1r radians between cathode 13 andfirst grids 21 or 23. There must therefore be provided a transit angleof about 31r radians between the first grids 21 and 23 and theirrespective second grids 19 and 25 to bring the phase right, since about%radians will be used up in the anode circuit. This total transit angleis small enough to insure that no serious debunching will occur.

The spacing between anodes 15 and 17 and their respective adjacent grids153 and 25 is not extremely critical. Though there is an optimum spacingthe minimum of the curve is broad, and the resulting transit angle maybe anywhere between 21 radians 4 and 11' radians without changing theefficiency apprecia v- The capacity between the various elements and itsrelation to the resistance due to the beam must be considered. The beamoffers the highest loading, and therefore the minimum of resistance, inthe region between the cathode and the first grids. Considered as lumpedcapacities the voltage across the gap will not be aifected appreciablyif the resistance of the gap is greater than the capacitive reactance,or otherwise stated the Q must be larger than unity if these elementswere in a resonant circuit. For reasonable values of spacing and size ofgrids this condition is fulfilled for both the input circuit and theanode circuit.

Another consideration which must be met in the construction of thisoscillator is that since the first grids 21 and 23 must necessarily haveradio frequency potential between them, therefore unless precautions aretaken, not only will the radio frequency power be conducted out theleads to the base of the tube, but also this loading will cause adifference in radio frequency potential at different points of the grid,whereas with no center connection the potential would be constant oruniform due to the radio frequency field. To overcome this problemeither of two solutions will suffice. As shown in Fig. 4 a loop 40 canbe provided connecting the two grids 37 and 38 which will intercept justsufiicient magnetic field to set up the same potential where the loop isfastened to the grids as the grids would normally assume due to thecapacity effects described above. This will level off the potentialsagain so that they will be uniform at all points along the grids. Thedirect current connection for supplying positive bias to these gridsfrom lead 24 should be taken off as a center tap half way around theloop 40. The same provision of a correcting loop 41 must be made also inthe case of the outer pair of grids 36 and 39. The optimum size of loopmust be determined finally by test but the first solution may be had byassuming a reasonable distribution of the magnetic field and choosingthe area of the loop to give the necessary voltage on the end of thegrid determined from capacity considerations.

An alternative and perhaps simpler method of accomplishing the result isto make the connecting leads in length before shorting them.

If symmetry is preserved there will be little leakage at the leads tothe base of the tube since the TEu mode here being dealth with has highattenuation along a concentric line. Only the asymmetrical componentwill be so propagated. Chokes could be included in the base of the tubeif desired.

The provision of screen grids 36 and 39 serves two purposes. First, itprovides a region between the grid and anode Where the transit time ofthe bunch of electrons after passing the first grids can be adjusted toarrive in the plate region in the proper phase, andsecondly it permitsthe electrons to enter the plate region with a relatively high energy,thus even though they stop at the plate they still have a comparativelysmall transit angle for a reasonable spacing between the outer grid andthe plate.

The anode face should allow the electron beam to enter through the facewithout appreciable interception and be trapped so that secondaryelectrons are discouraged from emerging back into the anode-outer gridgap and thus absorbing energy. This can be accomplished either byplacing a grid on the end of the anode or by mounting strip metal togive minimum interception to the beam. In either case the spacingbetween adjacent elements should be about the same as the spacing fromanode to the outer grid. This will insure a good radio frequency fieldwhich is necessary to extract the energy from the beam.

Fig. 4 shows the complete tube arrangement, including the evacuatedenvelope within which the anode cylinder 32 with its two vanes 33 and 34is inserted after assembly of the cathode 35 and the grids 36, 37, 38and 39 with the anode structure, 33 and 34. The grids are indicated asequally spaced merely for drawing convenience, and the respective ,pairsare shown joined by the correcting loops 40 and 41. For low voltagetubes the cathode 35 and the grids 36, 37, 33 and 39, are held with micadisks 42 and 43 at each end, and these disks 42 and 43 in turn are heldrigidly in the cylinder 32 forming the anode circuit. The anode cylinder32 could be mounted on the stem of the tube and after complete assemblyinserted into the metal shell 30 forming the vacuum envelope. If desiredto insulate the anode 32 electrically from the shell 3%, any suitableinsulating material such as a thin mica sheet 31 for example can beplaced between them.

The direct current base leads, 20 and 22 supply the cathode filament;grid lead 24 is center tapped to the correcting loop 40; while grid lead26 is center tapped to correcting loop 41. Lead 28 supplies anodepotential. The output lead for a low power tube of this type may consistof a concentric line such as shown, with inner conductor 46 and outerconductor 48 inserted through the top of the envelope 30 terminating ina loop 49 of proper size intercepting the magnetic field set up by theradio frequency currents. For high power tubes it would be desirable tohave a window, not shown, on the top of the tube to open directly into awave guide, not shown. The mode so generated if the tube fastened on tothe end of the guide would be the usual TEu in a circular guide or theTEoi in a rectangular guide.

An additional feature of this invention is the fact that such a tube canbe tuned, though over a limited range, say 10%, by squeezing theexternal envelope at the proper points, preferably say at points 59 and52 at the region of the two anode vanes 33 and 34 respectively. Motionis thus transmitted through both cylinders 30 and 32 resulting in acloser anode to outer grid spacing. Otherwise if the squeeze is appliedat points located at from the anode locations the anode to outer gridspacing would be increased. For many applications the metal cylinderscould be deformed permanently so as to be set for a given wave lengthand this can be done after the tube has been sealed off.

What is claimed is:

l. A microwave transit time oscillator tube comprising, an open endedconductive cylinder having two inwardly projecting anode vanes ofrectangular cross section diametrically opposed, with a separationbetween the adjacent ends thereof substantially equal to the thicknessof said vanes, a cathode disposed midway between said anode vanes, apair of grids disposed respectively between said cathode and each ofsaid anode vanes, a second pair of grids each disposed respectivelybetween one of said first named grids and the adjacent anode vane, andan evacuated envelope containing the aforesaid components.

2. A microwave transit time oscillator tube comprising, an open endedconductive cylinder having two inwardly projecting anode vanes ofrectangular cross section diametrically opposed, with a separationbetween the adjacent ends thereof substantially equal to the thicknessof said vanes, a cathode disposed midway between said anode vanes, apair of grids disposed respectively between said cathode and each ofsaid anode vanes, a second pair of grids each disposed respectively between one of said first named grids and the adjacent anode vane, and anevacuated envelope containing the aforesaid components, said envelopebeing adapted to be squeezed by means external to said envelope so as toalter the diameter of said cylinder in the region of said anode vaneswhereby the spacing between said anode vanes and the grids adjacentthereto may also be changed for elfecting a tuning of the frequency ofsaid oscillator.

3. A microwave transit time oscillator tube comprising, an open endedconductive cylinder having two diametrically opposed interior projectinganode vanes substantially centrally located, a cathode disposedsubstantially midway between the ends of said anode vanes, a pair ofgrids equispaced from said cathode on either side thereof, said gridsbeing connected to each other by a coupling loop adapted for maintainingthe end of the respective grid to which said loop is respectivelyconnected at substantially the same potential as the other portions ofsaid grid, a second pair of grids each being disposed respectivelybetween one of said first main grids and its adjacent anode vane, saidsecond named pair of grids being connected by a coupling loopsubstantially like that joining the first named pair of grids, and anevacuated envelope containing said components above described.

4. A microwave transit time oscillator tube comprising, an open endedconductive cylinder having two diametrically opposed interior projectinganode vanes substantially centrally located, adapted to oscillate in thefundamental mode, a cathode disposed substantially midway between theends of said anode vanes, a pair of grids equispaced from said cathodeon either side thereof, said grids being interconnected by a conductorof substantially one quarter wave length isolating the grids from eachother for alternating currents induced therein, a second pair of gridseach being disposed respectively between one of said first named gridsand its adjacent anode vane, said second named grids being connected bya conductor substantially like that between said first named grids, andan evacuated envelope containing said above-described components.

5. A microwave transit time oscillator comprising, an open endedconductive cylinder provided with two interior projecting anode vanesdiametrically disposed and adapted to oscillate in the fundamental modeat a wave length of approximately three times the diameter of saidcylinder, a cathode at a predetermined potential disposed midway betweenthe adjacent ends of said vanes, a pair of grids disposed equidistantlyfrom said cathode on either side thereof at a predetermined distancetherefrom whereby when said grids are provided with a positive bias of apredetermined magnitude relative to the potential of said cathode,electron bunches arrive at said grids and have a transit delay angle ofapproximately 1r radians, a second pair of grids respectively disposedbetween said'first pair of grids and their adjacent anode vanes at apredetermined spacing from said first named pair of grids, whereby whensaid second named grids are provided with a positive bias of a secondpredetermined magnitude and said first named grids are provided with apositive bias of said first predetermined magnitude, electron bunchesarrive at said second named grids from said first named grids and have atransit delay angle of approximately 2 radians, said anode vanes beingat a predetermined distance relative to said second named grids, wherebywhen said anode varies are connected to a potential source of apredetermined magnitude and said first named grids and said second namedgrids are provided with said first and second biases, respectively, thetransit delay angle of bunches of electrons arriving from said secondnamed grids to said anode vanes approximates '2- radians, and anevacuated envelope containing said components.

References Cited in the file of this patent UNITED STATES PATENTS

